Various kinds of optical elements and devices for directing light to desired direction, by making use of scattering, have been known heretofore. These optical elements and devices are employed as backlight sources of liquid crystal display and other applications.
According to one kind of these known optical elements and devices, a surface light source is used as a backlight source of liquid crystal display or the like, wherein light is introduced from one flank of a transparent material in the form of an extended plate with a light reflective element disposed at one side and light diffusing property is given to the other side of the transparent material.
In surface light sources using such light scattering and guiding light source, light is not scattered in three dimensions inside the transparent body and only the direction of light exit is spread by utilizing diffuse reflection and specular reflection near or at the surface of the transparent body or reflective element, with the result. Therefore, it is difficult, in principle, to sufficiently enhance the ratio of the scattering light that can be extracted from the light scattering and guiding device.
Where it is attempted to obtain a surface light source which receives light from one side and produces uniform brightness, some gradient must be imparted to the reflecting power of the reflecting element. This makes the structure of the light scattering and guiding device complex and bulky. Also, the manufacturing cost is increased. Therefore, where this type of light scattering and guiding light source device is used as a backlight source for a crystal display, some of the requirements, i.e., high brightness, high uniformity of brightness as surface light source, sufficiently reduced thickness, and excellent economy, must be sacrificed.
In view of these problems, some improved elements and devices for scattering and guiding light have been proposed. In particular, particles of material having a refractive index different from that of a transparent material in the form of an extended plate is dispersed in the transparent material to cause three dimensional scattering.
For example, in Japanese Patent Laid-Open Nos. 221925/1990 and 145485/1992, a backlight for a liquid crystal display is disclosed, wherein light is supplied to a flank of a light scattering and guiding plate and a light reflecting element is disposed at the side of the light scattering and guiding plate, while the opposite surface is used as a light exit surface.
When this type of light scattering and guiding element (an element intrinsically capable of scattering and guiding light) and a light source are conventionally combined to provide a light scattering and guiding device, some means are applied to get a flat intensity distribution of the light taken out of the light exit surface, wherein said means includes a graded density of particles dispersed in the light scattering and guiding element to increase the scattering power per unit volume at portions distant from the light source or a graded density of mesh or dot patterns of light diffusing ink applied to the backside of the light scattering and guiding element.
These additional technical means requires costs and provides a poor improvement in uniformity of brightness.
According to Japanese Patent Laid-Open No. 140783/1992, the thickness of a light scattering guide is increased at portions distant from the light source to make the brightness uniform, and according to Japanese Patent Application No. 102011/1993 (invented by the inventor of the present invention), two wedgeshaped light scattering guides having different scattering powers are combined to provide one light scattering and guiding device.
In the former method, however, the light incident surface of the light scattering guide has a small area, with the result that a transparent light guide is needed in order to compensate for this small area and a high light taking out efficiency from the light exit surface as viewed from the direction of the front surface is not be expected (It is guessed that a considerable portion of light incident on the transparent light guide body from the light source element is consumed without changing its direction to cross the light exit surface at a large angle.). In the latter technique proposed by the inventor of the present invention, the whole light scattering and guiding device exhibits light scattering power and a gradient is imparted to the cross sectional average scattering power, thereby improving the uniformity. Therefore, where the light exit surface is viewed from the front side, both light taking out efficiency and brightness are high. However, at least two light scattering guides of different scattering powers must be prepared and joined together. Hence, it cannot be said that the structure, the dimensions, and the manufacturing steps have been ideally simplified.